1. Technical Field
The present invention relates generally to austempered ductile iron (ADI) and an apparatus and method of forming the same. More particularly, the present invention relates to the localized formation of ADI. Specifically, the invention relates to a product having localized ADI, and the apparatus and method for controlling the heating and cooling of the item on which the localized ADI is formed.
2. Background Information
Austempering of ductile iron increases its hardness, abrasion resistance, ductility, toughness and fatigue resistance among other things. The ADI process has been used in the production of a wide variety of components, for instance, engine components such as crankshafts, camshafts, connecting rods; chassis components such as brackets, arms and knuckles; power train/drive line components such as gears, shafts, carrier housings and clutches, ring gears and pinions, and other gears as well; structural components such as brackets, side beams, rollers and so forth; and other wear components in various other applications. Generally, an iron part or a portion thereof is heated to an austenitizing temperature to transform it to austenite and to allow diffusion of carbon into the metal matrix of the ductile iron. The part is cooled from the austenitizing temperature at a rate sufficient to avoid formation of pearlite to a temperature above the martensite transformation temperature. The part is maintained at this target isothermal transformation temperature range for a time sufficient to form a metal matrix consisting primarily of ausferrite. Ausferrite is a matrix of acicular ferrite and carbon stabilized austenite, the latter also known as high carbon austenite. The rapid quenching and holding of the part at the transformation temperature range for a suitable period provides for the formation of ausferrite without the formation of pearlite or martensite.
This process has been performed by heating the entire part to the austenitizing temperature, as disclosed in U.S. Pat. No. 4,637,844 to Pfaffmann, and has also been performed selectively heating eccentric lobes of a camshaft to the austenitizing temperature while the remainder of the camshaft is not heated to this temperature, as disclosed in U.S. Pat. No. 5,028,281 to Hayes et al. In the latter process, only surface portions of the lobes are austenitized while the rest of the camshaft remains in a non-austempered condition. Hayes also indicates that the camshaft is quenched in a salt bath, which may include a mixture of sodium nitrite, sodium nitrate and potassium nitrate, to decrease the temperature rapidly enough to avoid the pearlite range, or alternately in a quench medium which may comprise an oil or a fluidized bed, the fluidized bed preferably including a heated granular solid medium having a gas such as air blowing through the medium. Another method, disclosed in U.S. Pat. No. 5,064,478 granted to Kovacs et al., includes uniformly heating the surface of a part by immersion in a molten metallic bath to form a desired thickness of surface austenite and thereafter quenching the heated cast iron part in a liquid quenching bath maintained at a temperature between 450° to 800° F. The Kovacs process does not allow for a specific localized hardening, but rather a hardening of the entire outer surface of the part.
These methods, as well as others, can require significant, expensive alloying or prior heat treatment of the ductile iron to achieve their purpose. Some of the alloying metals typically used are copper, nickel and molybdenum. For instance, these metals are generally added to provide sufficient hardenability for flame-based surface austempering processes. In addition, quenching baths such as a heated salt bath or oil bath may present health hazards or environmental hazards due to evaporation. Even lead baths have been used. Immersion of a part in a salt bath also requires the subsequent rinsing of the bath solution from the part. Immersion in a metallic molten bath (as in the method of Kovacs et al. noted above) is a relatively costly way of heating the part.
While the process of austempering, including surface austempering, is generally known, there still remains room for improvement in the formation of optimum metallurgical microstructures. In addition, there is a need for a more robust and cost effective method of locally austempering. The present invention addresses problems with the current art and provides an improved alternative.
Similar numbers refer to similar parts throughout the drawings.